Manufacture of formaldehyde



Jamz, w54 Ac. MCKHNNIS MANUFACTURE 0F FORMALDEHYDE Filed April 3, 1950 m N C.

Patented Jan. 26, 1954 MANUFACTURE OF FORMALDEHVYDE Art C. McKinnis, Long Beach, Calif., assigner to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application April 3, 1950, Serial No. 153,630

9 Claims.

This invention relates to the manufacture of formaldehyde, and in particular concerns an improved process for the production of formaldehyde by the partial oxidation of methane or natural gas.

It is known that formaldehyde may be formed by the partial oxidation of methane or natural gas, and a number of processes have been proposed for adapting the reaction to the production of formaldehyde on a commercial scale. All of such processes involve the use of oxygen or air as the oxidizing agent, and most of them employ metal oxides or nitrogen oxides as catalysts to promote the reaction. However, none of such proposed processes has met With commercial success, primarily because of the poor yield of formaldehyde realized and because of technical difliculties inherent in the manner in which the process is effected. Insofar as l. am aware, none of such processes has been able to produce formaldehyde in yields greater than 25 per cent of theoretical, based on the methane consumed. Furthermore, in those processes in which nitrogen oxides are employed as catalysts, corrosion and purification problems arise as a result of the nitrogen oxides occurring in the product in the form of nitric and/or nitrous acids. Attempts have also been made to oxidize methane to formaldehyde employing nitrogen dioxide as the sole oxidizing agent, but here again the yields of formaldehyde obtained were only about 20 per cent of theoretical, based on the methane consumed.

It is accordingly an object of the present invention to provide an improved process for the production of formaldehyde from methane or natural gas.

Another object is to provide a process whereby methane or natural gas is partially oxidized to formaldehyde by means of an oxidizing agent which can readily be regenerated for re-use in the process.

A further object is to provide a process `whereby methane or natural gas is oxidized to form formaldehyde in yields higher than those heretofore realized. Y

Other objects will be apparent from the followlng detailed description of the invention, and various advantages not specifically referred to herein will occur to those skilled in the art upon employment of the invention in practice.

I have found that the above objects and attendant advantages may be realized in a process whereby methane or natural gas is partially oxi- 'Ldized to formaldehyde by reaction with nitrogen dioxide at somewhat elevated temperatures over a relatively short period of time. More particularly, I have found that under certain conditions certain mixtures of nitrogen dioxide and methane or natural gas undergo combustion in the absence of oxygen to form formaldehyde in good yield, and that substantially all of the formaldehyde so formed may be recovered by quickly removing the combustion products from the combustion zone and cooling them to a temperature below the decomposition temperature of the formaldehyde product. In previous attempts to oxidize methane with nitrogen dioxide, the reactant gas mixture was subjected to temperatures of about L14N-680 C. for periods of time ranging from about 3 to about 18 seconds, and the yield of formaldehyde obtained was at best only about 20 per cent of theoretical, based on the methane consumed. Presumably, such low yields were at least in part due to the fact that the formaldehyde initially formed became decomposed or underwent further reaction during the relatively long period of time it was subjected to the elevated temperature. I have now found that yields as high as 40 per cent of theoretical,.based on the methane consumed, may be obtained by subjecting the reactant gas mixture to similar reaction temperatures for much shorter periods of time, e. g., 0.001-0.5 second. The invention thus consists in the process for the production of formaldehyde whereby a substantially oxygen-free gaseous reactant mixture of nitrogen dioxide and methane or natural gas is subjected to an elevated temperature suicient lto initiate combustion of the mixture, and the products of the combustion are removed from the combustion zone and are cooled to a temperature below the decomposition temperature of formaldehyde within about 0.5 second of their formation. Accordingv to a preferred embodiment of the invention, such ,process is effected simply by burning` a mixture of nitrogen dioxide and methane or natural gas in the substantial absence of oxygen while controlling the size and Shape of the ame so that the time of passage of the reactant gas therethrough is less than about 0.5 second.

The reaction whereby formaldehyde is formed according to the process of the invention may be represented by the equation:

(As is well known, nitrogen dioxide exists in equilibrium with its dimer, nitrogen tetroxide, and accordingly it will be understood that where the term nitrogen dioxide is employed herein such term refers to the equilibrium mixture of and water and is very readily oxidized back to nitrogen dioxide merely by contacting with air, so that the process may be operated in-a eontinuous cyclic manner with .continuous regeneration of the nitrogen dioxidei fe ag: tar1t,

mode of operation is illustrated diagrammatioale;

ly by Figure 1 of the accompanying drawing, and essentially comprises (1) aproduction step wherein a proportioned mixture of nitrogiegydif-v oxide and methane or natural gas is caused to vundergo 4combustion whereby. formaldehyde. nitriefoxide .endwater are...i..f. rrr.1.ed..according` to theebove reaction` equation, '(2) e separation step .whereby the nitric. oxide by-produot .and unreeoted Ymethane are seperated yfrom the formaldehydeand water dhd13) e regeneration step.4 wherein. thev gaseous mixture of nitrie ox- Vide andunreaotedmethane is mixed with OXY- gen.. or airwhereby thehitrio oxide is spontaneouely. oxidized to. nitrogen .dioxide dnd therel ie thus. obtaineda .recycle eee mixture .eomprieing nitrogen .dioxide ...and methane. Sueh .mixture is mixedwith sufeient methane tomeke up .for that consumedinthe production step and with euilieient. nitroeendioxide. to make up for inoidentel losses, epd the.. .resulting muiture is introduced into the. produetioh step. ee feed eee- .From the A.reaction equation Ygiven above .it Wouldappear that two moles or nitrogen dioxide should. bev provided. for. .eeen mole .oihiethe-ne or naturel gas, and. that-the reaetenteesmixture should. accordingly comprise. about 67. per dent by volume .of nitrogen dioxide end about 33 per eent by. .volume of .methane or. naturel ees... I have jecting the feed gas mixture to a reaction temperature between about 450 C. and about 700 C. for a period of time ranging from about 0.001 to about 0.5 second, as for example by passing the gas mixture through a heated tube of such length that the desired contact time is attained. Preferebly,` however, the eee mixture ie preheated and then passed to a burueryvhere itieiepited end allowed to burn with a free self-sustaining flame.

In order that the residence time of the combustion products within the flame be less than about 'Y 0.5 second, it is desirable that the flame take the form oa thi-nflat disc of only a few millimeters in thikness, i., e., it is desirable that the dimen- -Y sionfofutheil-amefin the direction of its propagation be small in comparison with its diameter. A flame of such 'shape is obtained when the Velocity @which the reactant gas mixture is supplied to found, however, thatwhen a Ysaemixture 0f. eheh v composition is. employed. substantially. ell. of.. the s methane is. oxidized .to .carbon monoxide. and. eerbon dioxidaand very .little formaldehyde ooelire inthe .product gas... Aooordinely, 1 prefer to. enoploya feed gas mixture conteinineen excess of methaneornaturel. gas, and, oertieulerly vone .in

which the mole `ratio of hydrocarbon to .nitrogen dioxide. Y.is between. about. .1:1 and. .about .5 2.1.- feed ees mixtures eonsietineonly. ormethepe or naturalgas and nitrogen dioxide, such mole ratios are attained inV eas mixtures .Consisting oiiroin about .50.to-about185. pereentby volume oimejdiE ane orneturel eas andirom about .5.0to about .1.5 percent by volume .of .nitrogendioxide .Aieed eesv .mixture consisting. o1v .about .6.5. per .cent by volume oi .methane Aor .natural ees and. ebout per. cent. by Volume of. nitrogen dioxidehee been iouhdparticularly satisfactory.. Ae aresult. of the use of such feed gasmixtures eontainine eXf cess .methanethe .produet comprises a eoneider able quantity of unreacted methane but no problem of separation arises since the .unreacted methane .and .nitric Voxide may he physically eeperated from the aqueous formaldehydeasa gas mixture. If desired, suchV gas mixture mayfbe further treated'toseparate the methane from the n itrieoxide,` but ordinarily the mixture is passed directly to .theregenerat-ion step where the nitric oxide.V .ie .oxidized with air. tofnitrogendioxide withoutainter erenee from theinethene.-

The: reeotih naar4 beeieeted .eimplyzihy sube temperature of. tbereeetentpsee mixture Supp ed to the. neme, i. e., .the preheet temperature,v be shape of the flame, and hence the reactiontime,

maybe tenderedbycontrolling.thepreheettemperetureand/.or the .flow velocity- Usually .itis desirable-to prelieet. the. eee Inixtureto eterne pereture of about4 1001140020... end to ediusttbe vthe desired dise-.shaped 11eme- Verioue -typee of burners. may illustratediii Fie)e be employed, `two oiy which are and .a produot. eee: outlet, -reepeeti Within body ilereperiorete .end I5 which.. .Ser-re. tol retaine-eee ermee peeking I6. .comprising aref-reo erle.,

. .Slia.- D... eelein orV glass. beads, ete. .Ret nine diee 1.14.- v

be of; the nature of.. e, perou.. ephreemroon'- s tructed, for: exampleo funglazed porcelain t but in: .order to minimize 4the prerieure drop the burner. it irv .reen-.l u eueily e-opetrpeted form. of .e eereen or a plete benne; dieerete .foretions- Reteiuipediee 'i 5, which forrriefxtbe .burnerv .ieeey .may lihewiee.i be a streep or` a; per- ,foretepleteor the. .like t iep eblyn nature of aporiuisl diaphragm eopetrfuetej or glazed porcelain, sintered;alguninaz` Vguarita .or silice/, porous .eerbon orerephite, ete: .Th

throught/nient.. .eeeti pees.; in which the reaction takes place. 1n-*operating tbietype. of: burne.r,.the feed eee mixto-rei -whieb has prefere 1y,l been preheeted/,is peeeed .in-io. the burner-throueihfinl-et. 'i2 end is .ignited v.on-.the downstream side of the burner face :Why-suitebleienitionfmeenenot shown. Therete of eas flow is controlled V so that .the dimension ,of the ilemein the dir. tionoffits propagation isemell enough that the. iii-ine oipeeseeeof .the reaction eas through the-neme .ie lee-e than about 0.5 eeeond; preferably about 0.001 Vto about 0.0.5 second, Tbefeombuetion products withdrawn from tbe burner drogen. outlet isi end ere cooled end treetedftoeepereie-the eompohepts thereof as is o.,e-.fuiiyfe.xo;leioed i The burner illustrated in cross-section in Figure 3 of the drawing makes use of the heat contained in the combustion products to preheat the feed gas. This burner comprises a tubular bod/ portion 2| having a closed end 22 and an open end 23 adapted to receive inlet conduit 24. Body portion 2| is also tted with a side-arm 25 which serves as a product gas outlet. Inlet conduit 2li is substantially coaxial with body portion 2| and extends Within the same to a point at least beyond side-arm 25. If desired, the assembly may be so constructed that the distance which inlet conduit extends within the body portion may be varied at will. In operating this type of burner, the feed gas mixture is passed into the inlet conduit 24 and is ignited at the open end thereof within the body portion 2| by heating closed end 22 of body portion 2| or by suitable igniting means, not shown. By suitably adjusting the rate of feed, the flame may be caused to take the form of a thin flat disc 25. The reaction gases pass through the name, reverse their direction of flow and pass through the annular space between body portion 2| and inlet conduit 22, and finally are withdrawn through the side-arm 25. As the hot product gases pass through the annular space between the body portion and the inlet conduit they serve to preheat the feed gas passing through the inlet conduit by heat-exchange through the walls thereof. By adjusting the distance between the open end of the inlet conduit and the side-arm, the amount of such preheating may be readily controlled.

While the main reaction which occurs under the conditions herein specified is expressed by the reaction equation given above, various side reactions also occur with the formation of carbon monoxide and carbon dioxide. Accordingly, the product gas will consist essentially of a mixture of formaldehyde, water vapor, nitric oxide, carbon monoxide, carbon dioxide, and unreacted methane. Substantially all of the nitrogen dioxide is converted to nitric oxide, so that at most only traces of unreacted nitrogen dioxide occur in the product gas. Treatment of such product gas for the separation of the various components may be effected in various ways. For example, the gas may rst be scrubbed with water whereby the formaldehyde and water vapor are selectively absorbed and thus separated from other components of the mixture. In order to prevent oxidation of the nitric oxide and the consequent formation of nitrous and/or nitric acid in the aqueous formaldehyde solution, the scrubbing operation is preferably carried out in the absence of oxygen, and the scrubbing Water should accordingly be boiled or otherwise treated to remove dissolved oxygen. The aqueous solution of formaldehyde thus obtained may have a concentration as high as 15 per cent byweight, and may be marketed as such or concentrated to higher strength by known methods. After removal of the formaldehyde from the product gas by scrubbing with water, the residual gas may be treated with aqueous calcium hydroxide whereby the carbon dioxide is removed in the form of calcium carbonate. The residual mixture of methane, nitric oxide and carbon monoxide is then mixed with oxygen or air whereby the nitric oxide is oxidized to nitrogen dioxide for re-use in the process. Since the reactant gas mixture should be substantially free of oxygen, the quantity of oxygen or air added to the residual mixture of unreacted methane and nitric oxide should be suicient konly to oxidize the amount ofnitric 6 oxide present to nitrogen dioxide, and is usually slightly less than this quantity. y

The side reaction whereby carbon monoxide is formed is accompanied by a second side reaction whereby the carbon monoxide so4 formed is oxidized to carbon dioxide. The rates of these two reactions are approximately equal, and accordingly the formation of carbon monoxide may be suppressed by providing an equilibrium amount of carbon monoxide in the reactant gas mixture. When operating in a continuous manner with regeneration of the nitrogen dioxide and recycling of the same along with the unreacted methane to the feed gas, it is not necessary to treat the recycle gas for the removal of carbon monoxide. The latter is allowed to accumulate until the equilibrium concentration is attained, whereupon further formation of carbon monoxide is suppressed, in accordance with mass action principles. By suppressing formation of carbon monoxide in this manner, somewhat improved yields of formaldehyde are obtained Other methods of treating the product gas for the desired separation of the various components may .be employed, and may be based on selective adsorption on solid adsorbents, selective absorption in various solvents, and selective chemical reactions.

The following examples will illustrate several ways in which the principle of the invention may be applied, but are not to be construed as limiting the invention.

A reactant gas mixture consisting of about '80 per cent by volume of natural gas and about 20 per cent by volume of nitrogen dioxide is preheated to a temperature of about 400 C. and is supplied to a burner of the type illustrated in Figure 2 of the accompanying drawing. The mixture is ignited at the burner face, and the feed rate is adjusted so that the arne takes the form of a thin flat disc of about 2 mm. thickness in the direction of its propagation and about l cm. in diameter. The residence time of the gas mixture within the flame is about 0.1 second. The products of the combustion are collected and countercurrently scrubbed with water from which the dissolved oxygen has been expelled by boiling. Analyses of the aqueous solution and of the residual gasshow that approximately 40 per cent Example II The burner employed is of the reverse-flow type illustrated by Figure 3 of the accompanying drawing. The body of the burner has an inside diameter of about 5 cm., and the inlet conduit has a diameter of about 4 cm. and extends inside the body for a distance of about 10 cm. beyond the outlet conduit and to within about 2 cm. of the closed end of the body. Ignition of the reactant gas mixture at the end of the inlet conduit'is secured by heating the closed end of the burner. The outlet conduit leads directly to a countercurrent scrubbing tower wherein the product gas is cooled and scrubbed with water from which dissolved oxygen has been expelled by boiling. The residual gas from the Water scrubber is then passed through an aqueous slurry of calcium hydroxide, whereby carbon dioxide is removed as insoluble calcium carbonate, and is thence lmixed with sufficient oxygen to react with the nitric oxide present to form nitrogen dioxide, vand-'is nnallyrlntrodueed into t..l.1e ,.tee.daasstamani..I After. the attainment ofrsteady stateeonditions, Ythe lifeboat-:temperature of. nthe .:reaetant gas before enteringftheznamezis :about 3.50919., with the gas beinesupplied tothe flameat alinear velocity of about 10cm/see.' The-name takes the formof a at .dise -tvhiehzis about .3 mm. thick in the direction ,of-itspropagation. The residence time 4.of; gt,l ie gaswithinythe .-iame is. thus about .-0.03 `sec nd... The reaotant gas .supplied to theburner .Consists-of. about 9.3.1 fmole of Vlresli-feed-gas, oonsistingiof .about pauper-cent by volumebf meth- .ane .andabout 57.4 per cent by volume ofnitrogen dioxide.and about 8.7 .moles Yof recycle gas-oonting of about ,1 -2:7 per .cent by `volume of ree -elated :nitrogen dioxide. 19.4 per cent by yolume ofunreaetednlethane, .and 68.9 percent by nie of carbon:Inonoxide..` VThe melee-.ratio .of ane to .nitrogen Adioxide .inthe reaetant sas us about 2:1.. Analysis of the aqueous solutalsen .from .the Wat-er scrubber.. showsitto onin about A0.088 mole of formaldehyde.' The residual eas Contains about v2.0 Vrnoles of unreaoted inethanaabout 6.0.rnoles1of .carbon monoxide, and about 1.0 mole of nitric oxide. VBy adding .0..5 tnole of v`.oxygento the residual eas. the nitric oxide `,is-oxidized to nitrogen dioxide to form the recycle gas of the aforementioned composition.

In the foregoing Example IL the reactant gas mixture-Supplied to the flame consisted of about 2dr-holes of Ymethane,.about 1.017 moles of nitrogen dioxide, and about 6.0 moles of carbon vmonoxide. As previously stated,.the methane to nitrogen dioxide ratio should vbe between about 1:1 and about :1... .eoui-lilniurn .amount of carbon-monoxidefvaries .depending upon the speoio C GlldtOnsnndel 'which the :reaction is elected, but usually amounts to about 3zrnoles per mole of methane., desiredthe advantage of 4providfns -thefequilibn oxide :in the reaotant eas mixture may be .realized .inl single-pass operation simply by .adding such .amount `from anrexternal source to the mixture ofl methane and 1nitrogen dioxide. .Qrdinarilu howeverfeontinuous operation Withfregeneration of..;t.he nitrogen dioxide. and recycling. of .the unreaoted .methane and Vrege.nenated V nitmeen .dioxideispreieried, infwhioh ease the equilibrium ,amount-of; carbon :monoxide is attained. .in the system simply by a owing itrto. accumulate-until ling, AIf desired, a multiplicity of burners may be mounted in a single enclosed space so `as to enable the production and collection of a large volurne pf product gas without necessitating a complicated manifold system..- Similarly, various .methods of treating the product eas Vfor the separation. of various components. or. groups of. cornponentsfthereoi may be employed, andY tbeform- .'aldehydeproduct. may be .concentrated or other- Y.wise .treated by any of the. -various-rnroeeduies )mourn inthe.. art.

-, amount of carbonv mon.- Y.

. lOthermodes of. applyingthe principle eff-my invention maybe employed instead of those .eX-- plained, .change being made in .the materials. -.or methods employed, provided the. step or steps stated by any vof the following claims, .or the equivalent of any such stated step or steps, be employed.

I, therefore, particularly point out anddistinctly claim as my invention:

1. The process for the production o f formaldehyde which comprises forming a substantially oxygen-free combustible mixture of nitrogen ldioxide and a hydrocarbon gas selected from `the class consisting of methane and natural gas, 4said mixture containing from about one to about flue moles of the hydrocarbon per mole of nitrogen dioxide, subjecting said mixture to a 'reaction temperature substantially equal to the Yname com.- bustion temperature of said mixture for. from about 0.001 to about 0.5.second and in the substantial absence of oxygen, and thereafter-sepa..- rating formaldehyde ,from the products of the reaction.

2. The process of claim 1 wherein the hydrocarbon gas is methane.

V3. The process of claim l wherein-the hyd-rocarbon gas is natural gas.

4. The process for the production of formaldehyde which comprises forming a substantially oxygen-,free combustible mixture of nitrogen :die oxide and methane containing from about one to about ve moles of methane per mole of nitrogen dioxide, supplying said mixture to a flame maintained by the combustion of. said .mixture .in the substantial absence of oxygen at avelooity such that the residence time of saidmixture within said name is between about 0.001 and .about.0.5 second, and thereafter separating formaldehyde from the products of the combustion.

5. A process for the production of formaldehyde which comprises forming a substantially oxygen-free combustible mixture of nitrogen dioxide and methane containing from about one to about ve moles of methane per mole of nitrogen dioxide, preheating said mixture to a temperature between about C. and about 400 C., supplying the preheated mixture to a flame main. tained by the combustion of said mixture .in the substantial absence of oxygen. Ythe rate :of supply being controlled so that the flame takes the shape .ofV .a thin llat disc having its .dimension inthe direction .of its propagation small with respect to its diameter and -so that thetme of passage .of said mixture through said name is between .about f .0.001 and about 0.5 second, and thereafter separating formaldehyde from the products .of the combustion.

6. The process of claim 5 wherein themixtllle is preheated by indirect heat exchange against the products of the combustion.

7. The process for the prodllution of formaldehyde which comprises forming a substantially oxygen-free combustible feed gas mixture of nitrogen dioxide and a hydrocarbon gas selected from the class consisting of natural gas and methane, said mixture containing from about one to about ve moles of the hydrocarbon per mole of nitrogen dioxide, supplying said mixture t0 a flame maintained bythe combustion of said mixture inl the substantial absence of oxygenat a velocity such that the residencetime of said mixture within said flame is between about .0.0.0la1ld about 0.5 second, `separati..ne .foilmaldehyde and water from .the products ofthe eoinbustionl adding-to the .residual cas sunieient oxygen-to oxidiac the nitric oxide contained therein to nitrogen dioxide, and employing said nitrogen dioxide in the formation of a further quantity of said feed gas mixture.

8. The process for the production of formaldehyde which comprises forming a substantially oxygen-free combustible feed gas mixture or' nitrogen dioxide, methane and carbon monoxide, said mixture containing from about one to about ve moles of methane per mole of nitrogen dioxide and an equilibrium amount of carbon monoxide, supplying said mixture to a ame maintained by the combustion of said mixture in the substantial absence of oxygen at a Velocity such that the residence time of said mixture within said flame is between about 0.001 and about 0.5 second, and thereafter separating formaldehyde from the products of the combustion.

9. The process for the production of formaldehyde Which comprises i'orming a substantially oxygen-free combustible feed gas mixture of methane, nitrogen dioxide and carbon monoxide, said mixture containing from about one to about five moles of methane per mole of nitrogen dioxide and an equilibrium amount of carbon monoxide, preheating said mixture by indirect heat-exchange against hot product gas, supplying the preheated mixture to a flame maintained by the combustion of said mixture in the substantial absence of oxygen at a velocity such that the ame takes the form of a thin at disc having its dimension in the direction of its propagation such that the time of passage of said mixture through said ame is between about 0.001 and about 0.5 second, separating formaldehyde, Water vapor and carbon dioxide from the products of the combustion, adding to the residual gas comprising unreacted methane, nitric oxide and carbon monoxide an amount of oxygen only sufficient to oxidize said nitric oxide to nitrogen dioxide, and employing the resulting gas mixture for the formation of a further quantity of said feed gas mixture.

ART C. McKINNIS.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,319,748 Bailey Oct. 28, 1919 2,128,908 Bludworth Sept. 6, 1938 2,196,188 Bone et a1 Apr. 9J 1940 OTHER REFERENCES Frolich et al., Jour. Am. Chem. Soc. (1928), vol. 50, pages 3216-21.

Bibb, Industrial and Engineering Chemistry, vol. 24, pages 10-12 (1937).

Groggins, Unit Processes In Organic Synthesis, 3rd edition (1943i), page 486, McGraw- Hill Book CompanyI New York. 

1. THE PROCESS FOR THE PRODUCTION OF FORMALDEHYDE WHICH COMPRISES FORMING A SUBSTANTIALLY OXYGEN-FREE COMBUSTIBLE MIXTURE OF NITROGEN DIOXIDE AND A HYDROCARBON GAS SELECTED FROM THE CLASS CONSISTING OF METHANE AND NATURAL GAS, SAID MIXTURE CONTAINING FROM ABOUT ONE TO ABOUT FIVE MOLES OF THE HYDROCARBON PER MOLE OF NITROGEN DIOXIDE, SUBJECTING SAID MIXTURE TO A REACTION TEMPERATURE SUBSTANTIALLY EQUAL TO THE FLAME COMBUSTION TEMPERATURE OF SAID MIXTURE FOR FROM ABOUT 0.001 TO ABOUT 0.5 SECOND AND IN THE SUBSTANTIAL ABSENCE OF OXYGEN, AND THEREAFTER SEPARATING FORMALDEHYDE FROM THE PRODUCTS OF THE REACTION. 